1. Field of the Invention
The present invention relates to a glass melting furnace and process which provide an increased glass production rate, thereby enhancing productivity, in addition to reducing pollutant emissions. More specifically, a sideport regenerative glass melting furnace of conventional design is provided with a first combustion zone at the batch charge end of the glass melting chamber which is side-of-port and/or overport fired and a second combustion zone at the glass discharge end of the glass melting chamber which is underport fired.
2. Description of the Prior Art
Glass melting furnaces typically operate at very high temperatures and burn large quantities of fuel to provide the required glass melting temperatures by direct heating. Fuel, typically natural gas or fuel oil, is mixed with sufficient air or other oxygen containing gas to provide complete combustion within the glass melting chamber. Glass batch comprising finely divided particulates is introduced at the charge end of the furnace, and melted, homogenized and refined as it is conveyed along the length of the furnace to the discharge end, where molten glass is discharged for further processing.
Ports are openings through the refractory walls into the interior of the glass furnace through which oxygen containing combustion gas is introduced into the furnace. In a regenerative glass furnace, the ports communicate with the regenerators which capture a portion of the heat contained in the glass furnace exhaust gases to preheat the oxygen containing combustion gas prior to its introduction into the glass melting chamber. Ports may be arranged in the side walls or end walls of the glass melting chamber. Sideport glass furnaces are provided with ports along the side walls of the glass melting chamber between the batch loading end and the opposite molten glass discharge end. Combustible fuel for melting the glass batch is provided by fuel nozzles or burners designed to burn a liquid fuel such as oil, or a fuel gas such as natural gas. At least one fuel nozzle is provided in conjunction with each port for combustion. Side-of-port fired furnaces are provided with fuel nozzles mounted at one or both sides of each port to direct fuel into the oxygen containing combustion gas stream. Underport fired furnaces are provided with fuel nozzles mounted underneath each port directing fuel into the oxygen containing combustion gas stream. Overport fired furnaces are provided with fuel nozzles mounted above each port directing fuel into the oxygen containing combustion gas stream.
Operating conditions within the glass melting chamber promote the oxidation of nitrogen in the combustion gas and fuel to NO.sub.x. At the high operating temperatures maintained in glass melting furnaces, substantially all the nitrogen oxidized in conventionally used combustion air forms NO. When exhaust gases containing NO react with cooler gases, NO is converted to NO.sub.x, principally NO.sub.2, a harmful pollutant. Large volume combustion sources such as glass melting furnaces are subject to strict pollution control regulations, and therefore, controlling pollutant emissions is an important factor.
Conventional endport fired glass furnaces have demonstrated that underport firing increases the efficiency of glass melting processes and reduces NO.sub.x emissions. Utilizing underport firing with sideport glass furnaces causes problems, however, because underport firing results in higher gas velocities and increased batch entrainment near the charge end of the tank causing higher particulate emissions.
Electrical glass melting and refining furnaces utilizing Joule effect heating have been developed. In most electrically heated furnaces, electrodes are submerged in the molten glass, and molten glass is heated as a result of the passage of electric current therethrough.
U.S. Pat. No. 4,029,489 teaches a glass melting furnace wherein glass is electrically heated from below its upper surface, and is heated from above its upper surface by fossil fuel combustion or heat reflectors. Sideport firing with burners extending through the ports is provided for the length of the furnace away from the charging region which is operated in a cold top melting mode. Heat barriers are provided between the discharge region and the charging region. As the tank is filled, fossil fuel firing is reduced, ports are closed, and full Joule effect heating is achieved, particularly toward the discharge end.
U.S. Pat. No. 3,856,496 teaches an endport fired regenerative glass melting furnace with overport firing in which the angle between each pair of burners is precisely adjusted to increase heat transfer to achieve complete combustion with a reduction in excess oxygen and to avoid impingement on the furnace walls.
U.S. Pat. No. 3,764,287 teaches a glass melting furnace having an induction medium comprising molten metal, the molten glass forming a buoyant layer on top of the induction medium and the furnace being underfired through the bottom wall with fossil fuel, the burners submerged in induction medium.
U.S. Pat. No. 3,836,689 teaches a glass melting furnace for electrically heating molten glass which has a plurality of controlled thermal zones to maintain the desired glass temperature profile. Electrodes are submerged in the molten glass, and the glass is heated by selective regulation of electric current provided to each electrode. U.S. Pat. No. 3,574,585 discloses an electric glass melting furnace utilizing submerged electrodes, the tank divided into at least two zones by a hanging transverse refractory wall.
A glass conditioner located between the melting furnace and glass forming machines for processing molten glass to control uniformity and homogeneity is disclosed in U.S. Pat. No. 4,424,071. The furnace is heated by combustion burners extending through sidewalls above the glass surface and electrical heaters below the glass surface.
U.S Pat. No. 4,328,020 teaches the injection of ammonia into a glass melting furnace exhaust stream to reduce NO.sub.x emissions.